Low-Temperature Fuel Cell Membrane Electrode Assembly Processing Techniques

Written by Colleen Spiegel on Apr 04, 2017. Posted in Component Information, Electrode Information, Membrane Information

The fuel cell stack consists of many layers, including:

• The Membrane Electrode Assembly (MEA)
• Gaskets
• Flow field plates
• End plates

An exploded illustration of the fuel cell layers in a low-temperature fuel cell stack are shown in Figure 1 below.

Figure 1: The fuel cell layers in a low temperature fuel cell stack.

The membrane electrode assembly (MEA) consists of the electrolyte, catalyst, and gas diffusion layers as shown in Figure 2 below.

Figure 2. The membrane electrode assembly (MEA) layers.

There are two standard methods of assembling the membrane electrode assembly (MEA) in low-temperature fuel cells:

(1) Applying the catalyst layer to the Gas Diffusion Layer (GDL), and then adding the membrane, or
(2) Applying the catalyst layer to the membrane followed by GDL addition.

The catalyst layer can be applied in one or two steps. For the first method, there are five common ways to prepare and apply the catalyst for the GDL/catalyst assembly:

1. Spreading: This method consists of preparing a catalyzed carbon and PTFE mixture by mixing and spreading it on using a heavy stainless steel cylinder on a flat surface. This leads to a thin and uniform layer where the Pt loading is related to the thickness.
2. Spraying: The electrolyte is suspended in a mixture of water, alcohol, and colloidal PTFE. This mixture is sprayed onto a wet-proofed carbon cloth, and the electrode is sintered between spraying to prevent the components from re-dissolving in the next layer. The electrode is then rolled to produce a thin layer of uniform thickness and porosity on the GDL/catalyst assembly.
3. Catalyst powder deposition: The Vulcan XC-72, PTFE powder, and a variety of Platinum on Carbon loadings are mixed in a fast-running knife mill under forced cooling. This is then applied to a wet-proofed carbon cloth. Applying a layer of carbon/PTFE also evens the surface of the paper and improves the gas and transport properties within the MEA.
4. Ionomer impregnation: The catalytically active side of the GDL is painted with a mixture of PFSA in a mixture of lower aliphatic alcohols and water. The catalyst and ionomer are mixed before the catalyst layer is deposited, to create a more reproducible uniform layer.
5. Electro-deposition: Electrodeposition impregnates the porous carbon structure with ionomer, an exchange of the cations in the ionomer by a cationic complex of platinum and electrodeposition of platinum from this complex onto the carbon support. This results in deposition of platinum only at sites accessed effectively by both carbon and ionomer.

For the second method, spraying, there are five common ways to prepare and apply the catalyst for the GDL/catalyst assembly:

1. Impregnation reduction: The membrane is ion-exchanged to the Na form and equilibrated with an aqueous solution of (NH₃)4PtCl₂ and a co-solvent of H₅O/CH₃OH. One side of the membrane is then exposed to dried PFSA in the H form, and the other side to aqueous reductant NaBH₄.
2. Evaporative deposition: (NH₃)4PtCl₂ is deposited onto a membrane through evaporation of an aqueous solution. Metallic platinum is produced by immersion of the entire membrane in a solution of NaBH₄. This method produces metal loadings of the order of < 0.1 mg Pt/cm² on the membrane/catalyst assembly.
3. Dry spraying: The reactive materials (Pt/C, PTFE, PFSA powder, and/or filler materials) are mixed in a knife mill, and the mixture is atomized and sprayed in nitrogen through a slit nozzle directly onto the membrane. The adhesion of this layer is good, but to improve the electric and ionic contact, the layer is then hot-rolled or pressed.
4. Catalyst decaling: Platinum ink is thoroughly prepared by mixing the catalyst and solubilized PFSA. The protonated form of PFSA in the ink is then converted to the TBA+ (tetrabutylammonium) form by the addition of TBAOH in methanol. The stability and spreadability of the ink is improved by adding glycerol to the mixture. The ink is then cast onto PTFE blanks for transfer to the membrane by hot pressing. When the PTFE blank is peeled away, a thin casting layer of catalyst is left on the membrane. In the last step, the catalyzed membranes are rehydrated and ion-exchanged to the H form by immersing them in lightly boiling sulfuric acid, followed by rinsing them in deionized water.
5. Painting: Pt ink is prepared in the same manner as the catalyst decaling method. A layer of ink is painted directly onto a dry membrane in the Na form and baked to dry the ink. The solvent is removed through drying in a heated vacuum chamber. The catalyzed membranes are re-hydrated and ion-exchanged to the H form by immersing them in lightly boiling sulfuric acid, followed by rinsing them in deionized water.

In addition to the methods previously listed, sputtering can also be used as a single step for catalyst preparation and application. Platinum can be sputter deposited onto one or both sides of the GDL. To enhance the performance, various coatings can be brushed on to the catalyzed surfaces of the membrane/catalyst assembly, such as a mixture of PFSA solution, carbon powder, and isopropyl alcohol. The solvent is removed by a vacuum chamber.

The final step for methods 1 and 2 is the addition of the membrane and GDL. Hot pressing is normally used for applying both of these layers. During hot-pressing, the membrane will dry out but becomes re-hydrated after insertion into the stack. It has been suggested in the literature that the membrane is treated with an H₂O/H₂O₂ solution heated to the boiling point, rinsed in deionized water, immersed in hot dilute sulfuric acid and treated several times in boiling water. This process removes impurities and traces of acid from the finished MEA.